(a) Field of the Invention
The present invention relates to a liquid crystal display having a plurality of gray voltages and driving apparatus and method thereof.
(b) Description of the Related Art
Liquid crystal displays (LCDs) include two panels having pixel electrodes and a common electrode and a liquid crystal (LC) layer with dielectric anisotropy, which is interposed between the two panels. The pixel electrodes are arranged in a matrix and connected to switching elements such as thin film transistors (TFTs), and supplied with data voltages through the switching elements. The common electrode covers the entire surface of one of the two panels and is supplied with a common voltage. The pixel electrode, the common electrode, and the LC layer form an LC capacitor in circuital view, which is a basic element of a pixel along with the switching element connected thereto.
In the LCD, voltages are applied to the two electrodes to generate an electric field in the LC layer, and the transmittance of light passing through the LC layer is adjusted by controlling the strength of the electric field to obtain the desired images. In order to prevent image deterioration that results from a prolonged application of the unidirectional electric field, the polarity of data voltages with respect to the common voltage is reversed for a frame, a row, or a dot.
However, due to the slow response time of LC molecules, it takes time for the voltage of an LC capacitor (referred to as “a pixel voltage” hereinafter) to reach a “target voltage,” which is the voltage required for a desired brightness. The amount of time needed to reach the target voltage depends on the difference between the currently applied voltage and the previously applied voltage of the LC capacitor. For example, when the target voltage is applied to the LC capacitor, the pixel voltage may not reach the target voltage while the switching element is turned on, if the difference between the target voltage and the previously applied voltage is large.
Dynamic capacitance compensation (DCC) is a technique that has been used to solve the above problem. The DCC technique utilizes the fact that the charging time becomes shorter as the voltage across the LC capacitor becomes larger. The DCC reduces the amount of time needed by the pixel voltage to reach the target voltage by applying a “data voltage” to a corresponding pixel. A data voltage is typically larger than the target voltage, and is herein used to refer to “the difference between the data voltage and the common voltage” by assuming that the common voltage is zero.
In a conventional gray-scale LCD, a black pixel voltage, which is the pixel voltage applied to the LC capacitor for displaying a black gray (i.e., the lowest gray), and a white pixel voltage, which is the pixel voltage applied to the LC capacitor for displaying a white gray (i.e., the highest gray), determine the upper and lower limits of the data voltages. That is, the data voltages are confined to a range between the black pixel voltage and the white pixel voltage. For example, the black pixel voltage and the white pixel voltage are the minimum and the maximum of the data voltages, respectively, in a normally black LCD (and vice versa in a normally white LCD).
In the normally black LCD, if a current pixel voltage represents a middle gray or a white gray and a target voltage is a black pixel voltage, a voltage smaller than the target voltage should be applied to the pixel voltage to reach the target voltage for a given period. However, it is impossible to apply such a voltage because the lower limit of the data voltage is the target voltage.
Likewise, if a current pixel voltage represents a middle gray or a black gray and a target voltage is a white pixel voltage, a voltage larger than the target voltage should be applied for the pixel voltage to reach the target voltage for a given time. However, it is impossible to apply such a voltage because the upper limit of the data voltage is the target voltage.
As a result, the DCC technique cannot be applied to a white gray or a black gray pixel to improve the charging time of an LC capacitor.
In particular, when displaying motion images with rapid gray changes, the failure to achieve the desired brightness severely deteriorates the image quality. This deterioration is more significant where the gray difference is large, such as when the gray is changed from a white gray to a black gray or vice versa. A method of achieving the desired brightness even when the gray difference is large would dramatically improve image quality in LCD applications.